1. Field of the Invention
The present invention relates to a magnetic bearing device for levitating and supporting an object under electromagnetic forces generated by an electromagnet.
2. Description of the Related Art
Displacement sensors for detecting the displacement of an object levitated by a magnetic bearing device include an eddy-current sensor, an inductive sensor, an electrostatic capacitive sensor, and an optoelectronic sensor such as a laser sensor. Of these displacement sensors, the eddy-current sensor and the inductive sensor are mainly used in turbomolecular pumps.
For use in corrosive environments, magnetic bearings and displacement sensors need to be covered with a protective material. Magnetic bearing devices which employ eddy-current sensors, inductive sensors, and electrostatic capacitive sensors can be covered with a protective material which may be synthetic resin such as Teflon or ceramics. Optoelectronic sensors such as laser sensors are required to be covered with glass which allows a laser beam to pass therethrough. Eddy-current sensors, electrostatic capacitive sensors, and optoelectronic sensors cannot be used in situations where displacement sensors need to be covered with a metal material. Inductive sensors can be used if they are to be covered with a nonmagnetic metal.
Protecting magnetic bearings and displacement sensors with Teflon, ceramics, glass, etc. poses problems in terms of fabrication process, cost, and mechanical strength. In addition, these protective materials may not be used in special environments where gas contamination is problematic.
It is customary to employ inductive sensors protected by a nonmagnetic metal in such applications. However, the carrier frequency of an inductive sensor produces a magnetic field that generates an eddy current on the surface of a nonmagnetic metal partition, resulting in a reduction in the sensitivity of a detected signal from the inductive sensor, i.e., a reduction in the s/n ratio thereof.
A magnetic bearing operates by passing a current through an electromagnet and levitating an object under electromagnetic forces generated by the electromagnet. If the magnetic bearing and an inductive sensor combined therewith are covered with a nonmagnetic metal, then since both the magnetic bearing and the inductive sensor are covered with one nonmagnetic metal partition, electromagnetic noise generated by the electromagnet and an eddy current produced on the surface of the nonmagnetic metal partition by the electromagnetic noise pass through the nonmagnetic metal partition, adversely affecting the inductive sensor. The nonmagnetic metal partition that protects the inductive sensor and the magnetic bearing is thus disadvantageous in that it makes magnetic levitation control difficult due to magnetic and electric noise applied to the inductive sensor.
FIG. 1 of the accompanying drawings shows a circuit arrangement of a control circuit for a conventional magnetic bearing device. As shown in FIG. 1, the control circuit includes an oscillator 1 whose output signal is supplied via operational amplifiers 2-1, 2-2, current-limiting resistors 3-1, 3-2, and a cable CB to a pair of series-connected displacement sensors Z1, Z2 which detect the displacement, in an X-axis direction, for example, of an object 5 levitated by a magnetic bearing MC. A potential (sensor signal) Eg at the junction between the displacement sensors Z1, Z2 is applied to a negative terminal of a differential amplifier 6, and a reference potential Es that is set up by reference resistors Ra, Rb is applied to a positive terminal of the differential amplifier 6. The differential amplifier 6 applies an output signal via a synchronous detector 7 and a phase compensating circuit 8 to a drive circuit 9.
As shown in FIG. 2 of the accompanying drawings, the drive circuit 9 comprises a controller 9-1 and a driver 9-2. The controller 9-1 controls the driver 9-2 according to a PWM process. The driver 9-2 supplies an output signal to an electromagnet coil 10 of the magnetic bearing MC.
In FIG. 1, a capacitor 4 is connected parallel to the displacement sensors Z1, Z2 to cause parallel resonance therewith.
If a protective plate of nonmagnetic metal is disposed between the displacement sensors Z1, Z2 and the object 5 and each of the displacement sensors Z1, Z2 comprises an inductive sensor, then the above problems arise, i.e., an eddy current generated by the protective plate of nonmagnetic metal causes a reduction in the sensitivity of a detected signal from the displacement sensors Z1, Z2, and the displacement sensors Z1, Z2 are adversely affected by a magnetic field generated by a current that is supplied to energize the electromagnet coil 10.